It has previously been proposed to use inertial guidance system for conducting surveys. One such system is described in an article entitled "The Application of Inertial Navigation Systems to Precision Land Survey", by S. R. Ellms and J. R. Huddle, pages 93 through 105, Navigation, The Journal of the Institute of Navigation, Summer, 1976, Volume 23, No. 2. In such prior systems, the emphasis was on position location rather than measurement of "deflection of the vertical". As mentioned above, the deflection of the vertical is a phrase used to designate the departure of the gravity vector from the gravity vector which would be expected from a mathematical model of the earth. When a point on the earth's surface is located between two portions of the earth's crust having significantly different densities, the gravity vector will be shifted slightly toward the more dense material, and this deflection of the gravity vector from the direction which would normally be expected is known as "deflection of the vertical". The deflection of the vertical is normally not very significant and rarely reaches a value above 50 or 100 arc-seconds, where there are 3600 arc-seconds in one degree. However, deflections of the vertical are of considerable interest to geologists and others who are interested in the location of bodies of ore, or other geological formations which are indicated by gravitational anomalies.
Returning to a consideration of prior navigation land survey methods, the method described in the article cited above included the mounting of an inertial system including a stable platform on a vehicle such as a jeep, aligning the inertial navigation system, and driving it from a first control point to a second control point. Between the two control points, the vehicle would be stopped at intermediate points to be surveyed, and at time intervals not in excess of predetermined limits, in order to eliminate accumulated errors. In addition, at each stop the inertial navigation system would determine the direction of the local vertical, and the stable element of the inertial system is releveled. At the end of the survey another control point would be reached, and the position errors for the intermediate points would be recalculated using the overall error in position, together with the time of surveying of the various individual intermediate points.
One of the serious short-comings of the prior system involved the releveling of the inertial guidance system at each stopping point. This produced a complex intermingling of errors associated with the inertial system with changes in the deflection of the vertical.
Accordingly, a principal object of the present invention is to avoid complex intermingling of inertial system errors with the change in the deflection of the vertical, in inertial guidance survey methods.